The combination of gas or liquid chromatography with infrared spectrometry has become widely used in chemical analysis. In particular, gas chromatography has been combined with Fourier transform infrared spectrometry to provide sensitivity levels and scanning speed which are superior to that obtainable with gas chromatography infrared spectrometry using dispersive instruments. Although the effluent from the gas chromatography column can be trapped and held for analysis, more commonly the stream is passed continuously through a light-pipe flow cell where an infrared beam passes through the flowing gas. The exiting infrared beam is then detected and analyzed.
The typical gas chromatograph flow cell accessory used in Fourier transform infrared spectrometry is a hollow tube or light-pipe with infrared transparent windows sealed to the ends of the tube. The characteristics of the light-pipe are crucial to the performance of the system. It is generally desirable to maximize the number of sample molecules that are in the infrared beam path while minimizing the radiation loss due to reflection and absorbence. The material in the light-pipe which contacts the gas must also be non-reactive. The light-pipes used in commercial instrumentation to meet these requirements are typically cylindrical glass tubes which have a thin coating of gold deposited on the inner surface. Gold is used because it is reflective, stable and inert. The glass light-pipes are surrounded and protected by a holder typically formed of metal, such as stainless steel. Infrared transmissive windows (e.g., potassium bromide) are mounted to either the ends of the light-pipe or to the holder with seals which seal off the ends of the light-pipe to prevent escape of the gases.
More recently, chromatography has been carried out utilizing supercritical fluids. Supercritical fluids have many attributes which allow high performance chromatography, including low mobile phase viscosity, high analyte diffusivity, and good solubility for a wide range of analytes. In addition, the observed chromatographic characteristics can be affected by changing the density of the mobile phase by changing the temperature or the pressure or both. Thus, a single supercritical mobile phase can be used to obtain a wide variety of separations without the time consuming column equilibration necessary in high performance liquid chromtography when changing mobile phase composition. Carbon dioxide is the most commonly used mobile phase in supercritical fluid chromatography.
Adequate detection has proven to be a significant instrument problem with supercritical fluid chromatography. Ultraviolet detection has been most commonly used to present, since many supercritical fluid chromatography phases are transparent in the ultraviolet region and most analytes studied contain ultraviolet chromophores. Flame ionization and fluorescence detection with capillary columns have also been used. However, these systems provide for essentially universal detection rather than specific detection. Fourier transform infrared spectrometry could yield both types of detection in real time, advantages which had been achieved with relatively high sensitivity in both gas chromatography and high pressure liquid chromatography. However, the conventional light-pipes used in gas chromatography Fourier transform infrared spectrometry or in liquid chromatography cannot be used for supercritical fluid chromatography because of the high operating pressures of the supercritical fluid.